This present invention relates to a system, including a telecommunications interface, and related method, directed to an adaptive envelope delay equalization that is used with bidirectional data communications lines in a transmission network, and more particularly, to a multi-variate system which has a microprocessor-based telecommunications interface having digital adaptive all-pass filter delay distortion equalization circuitry which is automatically variable for providing pre and post delay distortion equalization on the transmit and receive sides of dedicated telephone lines (e.g., standard 4-wire analog facility, although more than 4-wire tip and ring pairs are envisioned) and the data station termination at the customer premises equipment.